This invention pertains to the drying of wet feed gases. In particular, the invention provides various techniques for making a sweep gas for use in a membrane-based dehydration module.
Polymeric membranes have been shown to be effective for the removal of condensable gases, especially water, from otherwise desirable gas streams. A membrane module, containing a plurality of polymeric fibers, is typically used to dry a wet air stream, thereby providing dry air for use in a subsequent process.
The effectiveness of a membrane in gas separation depends not only on the inherent selectivity of the membrane, but also on its capability of handling a sufficiently large product flow. Gas permeates through the membrane due to the pressure differential between one side of the membrane and the other. Thus, to maintain the pressure differential, it is advantageous to remove the permeate gas from the vicinity of the fibers, after such gas has permeated through the fibers. Removal of the permeate gas maximizes the partial pressure difference across the membrane, with respect to the permeate gas, along the length of the module, thus improving both the productivity and recovery of the module.
The removal of the permeate gas is typically accomplished with a “sweep” stream, i.e. a stream of gas which carries the permeate gas away from the module. The sweep gas may also dilute the permeate gas, reducing its partial pressure, and further assisting in the removal of permeate gas from the module.
More details concerning sweep streams are provided in U.S. Pat. No. 7,517,388, the disclosure of which is incorporated by reference herein.
The present invention relates to the use of a membrane module for air dehydration. That is, the material of the membrane is chosen such that it separates air from water vapor, thereby drying the air. In this case, the dried air is the finished product of the dehydration membrane module, and it may be sufficient to divert some of the product gas for use as a sweep stream for the permeate gas produced by the module. U.S. Pat. No. 4,934,148, the disclosure of which is incorporated by reference herein, shows several examples of such a system.
In other cases, it is desirable to use a permeate stream from a second membrane unit to provide sweep gas to the permeate side of the first membrane, thereby avoiding the loss of product gas. U.S. Pat. No. 5,169,412, the disclosure of which is incorporated by reference herein, shows such a system.
In still other cases, the second permeate stream is recompressed and used as a sweep stream to remove the permeate gas from the first membrane. U.S. Pat. No. 5,226,932, the disclosure of which is incorporated by reference herein, uses a vacuum on the permeate side of the membrane to increase the partial pressure of the permeate gas and thereby enhance the effectiveness of the drying process.
What is needed is an effective and cost-efficient process to remove the permeate stream from a dehydration membrane.
In general, whether one uses the dried product gas as a sweep gas, or whether one uses ambient air, it is necessary to compress the gas before using it to sweep the permeate gas from the module. Alternatively, one could draw the sweep gas through the desired module by providing a vacuum at the low-pressure discharge side of the module. Both of the above approaches incur the cost of operating a compressor. Alternatively, it is possible to remove water from air by using a refrigeration dryer, but the refrigeration process also consumes power.
The present invention solves the above-described problems by providing an improved method and apparatus for generating a sweep gas for a dehydration module. The present invention minimizes the operating cost of generating a sweep gas, by avoiding the need for additional energy-consuming gas compression steps.